R. M. Deeley — Erosion by Rivers and Glaciers. 391 



terrace, though only occasional patches may have escaped subsequent 

 denudation. Neither does a flat plain always indicate river-work of 

 this kind, for many of the old river valleys of Norfolk and Suffolk 

 have been depressed at a comparatively recent date beneath the sea, 

 and have slowly been filled up with gravel, sand, and mud brought 

 down by the rivers and brooks, or removed by the waves from the 

 sea cliffs and driven inland. The well-known Broads are merely 

 isolated patches of what were extensive estuaries. Here, however, 

 we are concerned with the effects resulting from the combined 

 action of sea and river. 



The changes which occur from season to season in the volume 

 of water passing down stream depend not only upon variations in 

 the rainfall, but likewise upon the porosity of the rocks over which 

 the water flows. Indeed, it often happens that so large a percentage 

 of the rain sinks immediately into the ground, and rises again as 

 springs in the beds of the principal rivers, that the volume of water 

 passing seawards is very considerable even during dry weather. 

 However, although the river channels may be somewhat rapidly 

 and continuously eroded, the general land-surface occupied by 

 insoluble rocks does not suffer very rapidly. But the case is 

 different if the rock should be a soluble one, for the springs are 

 then exceedingly hard and great quantities of material pass to the 

 sea in an invisible form. The ease with which rainwater finds its 

 way into the ground, and after passing for long distances beneath 

 it again comes to the surface, has, we shall find, an important bearing 

 upon one aspect of glacier erosion. Vegetation also has an important 

 effect upon the rate of erosion, for it prevents the loose soil from 

 being readily washed down the slopes, and in estuaries accelerates 

 the rate at which silting up takes place. In this manner climatic 

 effects tell, for in high latitudes the land, which is often bare of 

 vegetation during a considerable portion, if not the whole, of tlie 

 year, is softened and loosened by frost, and the soil-cap slides 

 bodily down the hill slopes, producing one vai-iety of what we know 

 as "trail" and " underplight." Erosion effected in this way can 

 scarcely be called river erosion : by this we mean the scouring 

 action of the current, largely due to the grinding action of the 

 stones and grit it contains, on the sides and bottom of the channel. 

 Even when the rocks are soft the erosive power of the stream is 

 largely due to the pebbles carried along the bottom. 



Now, if from any cause the supply of debris brought down from 

 the hills by lateral streams should be greater than the river can 

 carry away, then instead of erosion taking place and gravel terraces 

 being formed the valley may become choked to a great depth with 

 rough gravel, mud, etc. Jn cases where the supply of debris 

 brought down in this way is not sufiicient to actually choke the 

 main stream, as is the case in the Indus Valley, the lateral streams, 

 where they debouch on the plains, spread out immense fans of gravel 

 on the alluvial deposits. However, in course of time the river cuts 

 into them and hurries the material towards the sea. 



The levelling action of rivers is well shown by some of the 



